1. Field of the Invention
The present invention relates to a pattern forming method using a phase shift mask, and more particularly, to a pattern forming method of carrying out double exposure of a single photoresist using a phase shift mask.
2. Description of the Background Art
Significant improvement in both integration degree and miniaturization of semiconductor integrated circuits have been made in recent years. With such improvement, reduction in size of circuit patterns formed on semiconductor substrates (hereinafter simply refereed to as wafers) have been rapidly achieved.
Particularly, the photolithography technique has been widely recognized as a basic technique of pattern formation. Therefore, various development and improvement of the photolithography technique has been made. However, patterns become increasingly smaller, and the need for improvement in resolution of patterns has been increased.
This photolithography technique is a technique of transferring patterns of a mask (an original picture) onto a photoresist applied on a wafer and patterning a lower film to be etched using the pattern-transferred photoresist. The photoresist is subjected to development after patterns are transferred thereto. A photoresist of a type in which a portion exposed to light is removed by development is called a positive photoresist, while that of a type in which a portion not exposed to light is removed is called a negative photoresist.
Generally, the resolution limit R (nm) in the photolithography technique using the reduction type projection printing is given by the following equation: EQU R=k.sub.1 .multidot..lambda./(NA)
where .lambda. is a wavelength (nm) of light for use, NA is numerical aperture of a lens, and k.sub.1 is a constant depending on a resist process.
As can be seen from the above equation, in order to improve the resolution limit R, that is, to obtain small patterns, it is possible to reduce values of k.sub.1 and .lambda. and increase a value of NA. More specifically, it is possible to reduce both a constant depending on a resist process and a wavelength and to increase NA.
However, improvement in light sources and lenses is technically difficult, and both reduction in wavelength and increase in NA causes reduction in the depth of focus .delta.(.delta.=k.sub.2 .multidot..lambda./(NA).sup.2), resulting in degradation of the resolution.
Then, shrinking of patterns by improvement of photomasks instead of both light sources and lenses has been studied. In recent years, phase shift masks have attracted attention as photomasks of a super-resolution technique for improvement in the resolution of the patterns. The structure and principle of the phase shift masks will now be described in comparison with the ordinary photomasks. The phase shift masks of Levenson and halftone types will be described.
FIGS. 24A, 24B and 24C are diagrams respectively showing a cross section of a mask, an electric field over the mask and a light intensity at a wafer in the case of an ordinary photomask. Referring to FIG. 24A, the ordinary photomask is structured such that a metal mask pattern 103 is formed on a glass substrate 101. An electric field over such a photomask is spatially pulse-modulated by metal mask pattern 103 as shown in FIG. 24B.
Referring to FIG. 24C, however, with shrinking of patterns, exposure light passing through the photomask also reaches around an unexposed region (a region which is shielded from exposure light by metal mask pattern 103) on a wafer due to optical diffraction. Therefore, the unexposed region on the wafer is also irradiated with exposure light, causing reduction in a light contrast (the difference in light intensity between exposed and unexposed regions on the wafer). As a result, the resolution is degraded, making it difficult to carry out transfer of small patterns.
FIGS. 25A, 25B and 25C are diagrams respectively showing a cross section of a mask, an electric field over the mask and a light intensity at a wafer in the case of a phase shift mask of Levenson type. Referring first to FIG. 25A, the phase shift mask includes an optical member 105 called a phase shifter in addition to the components of the ordinary photomask.
More specifically, a chromium mask pattern 103 is formed on a glass substrate 101 so that light transmitting regions and light shielding regions are provided, and phase shifter 105 is provided every other light transmitting region. This phase shifter 105 serves to change a phase of light passing therethrough by 180.degree..
Referring to FIG. 25B, since phase shifter 105 is provided in every other light transmitting region as described above, such an electric field is formed over the mask that light passing through the light transmitting regions of the phase shift mask is alternately reversed in phase by 180.degree.. Thus, since light at adjacent exposed regions is opposite in phase, light will cancel each other in the overlapping region of exposure light being opposite in phase, by optical interference.
As a result, as shown in FIG. 25C, a light intensity is reduced at the portion between exposed regions on the wafer, whereby sufficient difference in light intensity between exposed and unexposed regions on the wafer can be achieved. Thus, the resolution can be improved and transfer of fine patterns can be realized.
FIGS. 26A, 26B and 26C are diagrams respectively showing a cross section of a mask, an electric field over the mask and a light intensity at a wafer in the case of a phase shift mask of halftone type. Referring first to FIG. 26A, in the phase shift mask of halftone type also includes an optical member 106 called a phase shifter as in the case of that of Levenson type described above.
However, optical member 106 is formed only on a semitransparent film 103 on a glass substrate 101 so that a two-layer structure consisting of phase shifter 106 and semitransparent film 103 is provided. This phase shifter 106 serves to change a phase of light passing therethrough by 180.degree. as in the case of the above described phase shift mask of Levenson type, and semitransparent film 103 serves to attenuate an intensity of exposure light without completely blocking the exposure light.
Referring to FIG. 26B, since the two-layer structure consisting of phase shifter 106 and semitransparent film 103 is provided as described above, such an electric field is formed over the mask that light passing through a semi transparent region and a light transmitting region of the phase shift mask is alternately reversed in phase by 180.degree., wherein a light intensity at one phase is lower than that at the other phase. More specifically, light is reversed in phase by 180.degree. as a result of passing through phase shifter 106, and is attenuated in intensity as a result of passing through semitransparent film 103 so that a photoresist with a prescribed thickness is left after development. Since light at adjacent exposed regions is opposite in phase, light cancels each other in the overlapping region of exposure light being opposite in phase.
As a result, as shown in FIG. 26C, a phase is reversed at an edge of a pattern, so that a light intensity can be reduced at the edge of the pattern. Consequently, the difference in intensity between exposure light passing through semitransparent film 103 and exposure light not passing therethrough is increased, and resolution of a pattern image can be improved.
The phase sift masks for the super-resolution technique are of various types such as Levenson and halftone types. Effective methods for formation of hole-shaped patterns in the super-resolution technique includes exposure using the above described phase shift mask of halftone type, and exposure using a phase shift mask of auxiliary pattern type. However, the effects obtained by these exposures are not sufficient, and it has been considered that formation of holes having an opening diameter of 200 nm is a practical limit in the case of exposure by KrF light with a wavelength of 248 nm, for example.
On the other hand, an opening diameter of holes required for devices has been already in the range from 150 to 100 nm, and formation of such small holes has been carried out by a diameter reducing technique using film formation, etching and the like. However, this technique requires a number of steps such as film formation and etching, causing significant increase in manufacturing cost of devices.
A countermeasure of the above problem is disclosed by Japanese Patent Laying-Open No. 5-197126. The method therein will now be described as a conventional pattern forming method using a phase shift mask.
FIG. 27 is a plan view schematically showing a structure of a phase shift mask for use in the conventional pattern forming method, and FIG. 28 is a schematic cross section taken along the line G-G' in FIG. 27. Referring to FIGS. 27 and 28, a phase shift mask 210 is structured such that only phase shifters 203a and 203b are formed on a quartz substrate 201. A surface of quartz substrate 210 is equally divided into first and second regions 201a and 201b. A plurality of phase shifters 203a extending in parallel in the longitudinal direction (the direction Y in the figure) are provided in first region 201a, and a plurality of phase shifters 203b extending in parallel in the direction (the direction X in the figure) perpendicular to the longitudinal direction are provided in second region 201b.
As exposure, double exposure of a resist is carried out using the above described phase shift mask 210. More specifically, as shown in FIG. 29, a photoresist 211 is first subjected to first exposure. Thereafter, photoresist 211 is subjected to second exposure with phase shift mask 210 being moved such that second region 201b in the first exposure overlap first region 201a in the second exposure. Thus, exposure light is directed to photoresist 211 such that phase shifter 203b of second region 201b in the first exposure intersects phase shifter 203a of first region 201a in the second exposure.
In the exposure using phase shift mask 210, fine dark lines (regions with the lowest light intensity) are formed in the portions of photoresist 211 corresponding to shifter edge portions 203c and 203d of phase shifters 203a and 203b, as shown in FIG. 30. Therefore, if double exposure is carried out such that the fine dark lines intersect each other, small dark points are formed in intersections 207 of the fine dark lines.
If negative photoresist 211 is brought into a photo-sensitive state in the above described manner, hole patterns with a small diameter is formed at small dark points 207.
In the technology described in the above document, exposure with an i-line (a wavelength of 365 nm) makes it possible to form holes with an opening diameter of 200 nm.
In the conventional pattern forming method, however, a phase shift mask having no light shielding film is used. Therefore, hole patterns with a small diameter can be formed only when the distance between fine dark lines is large enough, that is, when the lines are isolated from each other. More specifically, for phase shift mask 210 shown in FIGS. 27 and 28, the distance L.sub.1 or L.sub.2 between light transmitting regions needs to be equal to or more than a so-called coherence length (up to 1.0 .mu.m for i-line and up to 0.7 .mu.m for KrF light). Therefore, the distance between hole patterns formed in the photoresist is necessarily increased, so that dense holes (holes with a small distance therebetween) for memory devices or the like such as DRAMs (Dynamic Random Access Memories) cannot be formed.
In order to make sure of the foregoing, we carried out the following simulation using a phase shift mask having no light shielding film.
First, a one-dimensional mask (i.e. a mask with a line shaped pattern) in which a phase shifter 303 extending straight on a transparent substrate 301 was formed as shown in FIGS. 31 and 32 was used as phase shift mask. Note that FIG. 32 is a schematic cross sectional view taken along the line H-H' of FIG. 31.
Exposure was carried out with focus offsets .DELTA.F of 0 .mu.m and 1.0 .mu.m for both dense patterns with a distance of 0.5 .mu.m between shifter edges of the phase shift mask (that is, the distances L.sub.3 and L.sub.4 are 0.5 .mu.m each) and sparse patterns with a distance of 2.0 .mu.m therebetween (that is, the distances L.sub.3 and L.sub.4 are 2.0 .mu.m each) to measure a light intensity at a wafer. The exposure was carried out with numerical aperture NA of 0.45 and coherency .sigma. of 0.50.
As a result, when exposure light was focused with a focus offset .DELTA.F of 0 .mu.m, a light contrast was large enough to produce sufficiently dark lines for both the above-mentioned distances of 0.5 .mu.m and 2.0 .mu.m as shown in FIGS. 33 and 34, respectively. More specifically, a light intensity at a shifter formation region was sufficiently lager than that at a shifter edge portion (a portion around 0.4 .mu.m in the figure).
Furthermore, even when exposure light was defocused with a focus offset .DELTA.F of 1.0 .mu.m, a light contrast was large enough to produce sufficiently dark lines for the above-mentioned distance of 2.0 .mu.m, as shown in FIG. 35.
For the above-mentioned distance of 0.5 .mu.m, however, a light contrast was reduced so much with a focus offset .DELTA.F of 1.0 .mu.m that the resultant image was not sharp enough, as shown in FIG. 36.
It is herein assumed that a pattern shape of a resist is determined by a slice level S (shown by a dotted line) in the figures. Then, in the case of the sparse patterns with exposure light being focused with a focus offset .DELTA.F of 1.0 .mu.m, since there is a region having a light intensity lower than the slice level S as shown in FIG. 35, patterns are formed in the resist. In the case of the dense patterns with a distance of 0.5 .mu.m therebetween, however, when exposure light is defocused with a focus offset .DELTA.F of 1.0 .mu.m, there is no region having a light intensity lower than the slice level S as shown in FIG. 36, so that patterns cannot be formed in the resist.
Note that the slice level S herein indicates a value of a light intensity used as a reference of determination whether a resist is removed or not by development (or whether a resist is left or not by development). More specifically, a region exposed to light having a light intensity equal to or higher than the slice level S is removed by development in the case of a positive resist, while is left by development in the case of a negative resist.
It can be seen from the result of the above simulation that, for the phase shift mask having no light shielding film, sufficient depth of focus DOF cannot be assured in the case of dense hole patterns such as those with a distance of 0.5 .mu.m therebetween and small hole patterns cannot be formed with exposure light being even slightly defocused.